Commonly this is pointing to the fact that the final day that the employee worked (and should be paid for) is after their last payment. Sometimes this happens when an employee is paid their final pay a few days before their last day of work, or more rarely if termination pay is paid out a couple week's after the last day worked.
In the example below, an employee's last day was the 5th, but his final pay and hours were paid out in the previous pay period. The date in the Last Day for which Paid box should always be before the date in the final pay period.
If the last day is correct, then the last pay period must be changed. We need to edit the information to reflect when the amount was earned, rather than when it was paid.
Note if the Pay History has reached maximum number of entries (25 for semi-monthly, 27 for bi-weekly and 53 for weekly), please delete the entry with the earliest pay period ending date and then you will be able to Add Pay History.